Screen printing processes employing thermoplastic inks or hot melts are well known. An early example of such a method is disclosed in U.S. Pat. No. 2,731,912 issued to Welsh wherein a process is described for screen printing or decorating objects such as ceramic ware by heating a thermoplastic color compound and forcing it through a screen having a pattern to deposit a design onto the ceramic surface. Another example of such a screen printing process is disclosed in U.S. Pat. No. 3,577,915 issued to Thompson et al wherein specific thermoplastic polyethylene ink compositions are employed and such compositions are forced through a heated screen to permit the viscous thermoplastic polyethylene to satisfactorily pass through the screen. It is common in such screen printing processes for a hot thermoplastic ink to be forced through a heated screen. The hot screen has nonporous areas that prevent the hot melt ink from going through the screen and porous areas that allow the ink to go through the screen. The porous area of the screen represents the design or letters to be transferred. The screen is manufactured from a material, such as metal wire mesh, that is able to withstand temperatures above the melting point of the hot melt ink. As the printing screen is placed into contact with the substrate to be imaged, the hot melt ink is forced through the screen and the ink solidifies upon the substrate. Mechanical means can be used to aid the thermoplastic ink to flow through the screen. For instance, rollers, squeegees, and the like, have been used to transfer a thin printing film of hot melt onto the substrate through the screen. The printed surface is then removed from the screen and another blank surface to be imaged is placed in contact with the screen on a continuous basis, for instance by employing a rotary screen apparatus, and the cycle is repeated.
Other examples of hot melt printing inks and methods of use in hot melt screen printing include U.S. Pat. Nos. 3,275,494; 3,294,532; 3,399,165 and 4,018,728. These patents are merely representative and by their listing here it is not represented that they are the most pertinent prior art.
A number of process considerations must be taken into account in order to obtain satisfactory results with hot melt ink screen printing. For instance, the hot melt ink must remain at a temperature above its melting point during the operation in order to possess sufficient viscosity characteristics to flow properly through the porous portions of the screen. Printing temperatures are extremely important because certain ink compositions such as heat sensitive or thermosetting hot melts tend to cross-link or are otherwise adversely affected by high temperature. This usually requires either operation at very controlled temperatures or coarse screens are desired in order to permit a fast transfer of the hot melts through the screens during the printing process. Such coarse screens obviously affect the type of image and thus, fine images are not capable of being achieved with such coarse screens. In addition to the control of process conditions, a number of problems frequently occur. For instance, if the printing temperatures are not able to be safely maintained, high ink viscosities are usually encountered and poor printing results because the ink is not sufficiently thin to flow through the porous screen. On the other hand, if low viscosities are achieved, the ink will flow too freely and poor printing results. Frequently, upon operating at high temperatures, cross-linking of polymeric inks causes plugging of the screen and, of course, this adversely affects the ability to print as well as the resulting images. Frequently the screens themselves must be heated in order to permit the hot melt inks to flow-through the printing screen. Such processes require screens, therefore, which resist destruction by heating. Such a limitation restricts the printing process because certain screen materials control the screen pore size which in turn affects the quality of the printing.
The demand for higher quality and high performance inks has placed more severe constraints upon the processing of hot melt inks. For instance, even where it would be desirable to use a polymeric ink because of its resistance to physical or chemical attack for a number of end uses, it has in the past been impractical if not impossible to screen print with a highly viscous polymeric material. Such a material with a high viscosity is not capable of being processed through a screen with any degree of precision in order to provide quality printing. Thus, while there are high performance thermoplastic or thermosetting compositions which would very desirably be employed as potential inks in screen printing, their use has been prevented because screen printing processes and techniques are not available to handle such high performance and high viscosity inks. It would be very desirable to provide a process whereby such high performance and high quality thermoplastic and thermosetting materials may be employed for screen printing to meet the demands of industry today.
It will be appreciated by a person of ordinary skill in this art that the screen printing industry, particularly as it pertains to printing with hot melt thermoplastic compositions, is in need of further improvements. Against the brief background of prior art presented above, there are a number of process parameters which make it difficult to control the printing process with hot melts and limit the nature of the thermoplastic inks which have been employed. Further improvements are needed in order to utilize a broader class of thermoplastic and thermosetting ink compositions at such speeds which render the screen printing process economical. It would be very desirable to provide screen printing processes which involve faster ink penetration and flow-through of thermoplastic melts and yet enable fast set characteristics consistent with printing operations. In view of the high cost of certain desirable polymers, quality printing is desired with the least amount of ink transferred through the screen. Consideration of all the above factors leads to the conclusion that further improvements in the screen printing of hot melt compositions are needed.